In the field of motor vehicles, there has been a steady development of various types of interior fittings and equipment for the safety and comfort of the occupants of a vehicle.
Concerning seat belts, many types of seat belt arrangements have been developed. When a vehicle frontal collision occurs, a vehicle seat belt is effective to fasten and protect an occupant against injuries. A seat belt is normally wound around an axis in a seat belt retractor. There is, however, a part of the seat belt that will be paid out in the event of a collision due to the fact that the seat belt is normally not tightly wound around the seat belt retractor axis. This part of the seat belt results in a slack which will cause the occupant that is fastened by the seat belt to travel a small distance forwards with respect to the vehicle before the seat belt stops the forward motion of the occupant. This means that the occupant will attain a certain velocity with respect to the vehicle before being caught by the seat belt. This may cause injuries to the occupant.
In order to solve this problem, it is previously known to provide the seat belt with a pre-tensioner which winds back the seat belt retractor axis, thus tightening the seat belt, when a collision occurs. The pre-tensioner is connected to a suitable retardation sensor, and when the vehicle is subject to a retardation which exceeds a predetermined limit, a control signal will be fed to the pre-tensioner. Upon reception of the control signal, the pre-tensioner tightens the seat belt essentially before the fastened occupant has begun to move forwards, thus eliminating the small slack. Thus, the injury preventing function of the seat belt is enhanced. The pre-tensioner may operate to tighten the seat belt by means of a spring or by means of pyrotechnics. The retardation sensor may be the same that generates a control signal to activate one or more airbags installed in the vehicle.
Using the pre-tensioner described above, the fastened occupant is subject to an abrupt impact force in the event of a frontal collision. Therefore, it is previously known to use a seat belt which, in the event of a collision, permits the fastened occupant to travel forwards with respect to the vehicle in a controlled manner. In this manner, an even higher degree of safety is provided. The occupant is thus still retained, and is always restrained by the seat belt which has a retaining force but which also permits a forward decelerated motion.
The patent document U.S. Pat. No. 6,213,510 teaches an arrangement in which a seat belt runs through a restraint device which mechanically actuates an energy-absorbing device for clamping the seat belt and then stopping an occupant during a collision. The energy-absorbing device is constituted by a plastically deformable plastic strip, allowing the occupant to travel forwards in a controlled manner before being stopped completely.
A disadvantage with the arrangement according to U.S. Pat. No. 6,213,510 is that it relies on an energy-absorbing element in the form of a compressible strip. Due to the fact that the belt pay-out for a front seat occupant, for example, should be as much as 45 cm in order to provide adequate protection, the housing which accommodates the strip must be of equal length. This is a problem since it requires a relatively large space in a vehicle.